The present invention relates to an electrophotographic copying process apparatus, and more particularly, to such a process and apparatus for performing the process for forming an electrostatic latent image of a high contrast voltage on an electrophotographic photosensitive member.
It is well known in the art that an electrophotographic photosensitive member has a basic construction including a conductive supporter, a photoconductive layer and a transparent insulator layer in sequentially laminated relationship. By way of example, an electrophotographic process for forming an electrostatic latent image on the photosensitive member has been known in which after a primary charging, a secondary corona charging is provided by a sinusoidal wave a.c. corona discharging during an imagewise exposure and subsequently a uniform exposure is applied to form an electrostatic latent image (Japanese Patent Publication No. Sho 42-23910).
However, the above electrophotographic process of the prior art, in which a secondary corona charging is effected by a sinusoidal wave a.c. corona discharging, has the characteristics of the corona discharging in that even if a positive and a negative voltage-time integration value of a sinusoidal wave alternate current applied to a corona charger are equal, the negative corona charging is more easily effected than the positive one, resulting in that the charging is apt to incline toward the negative side. Accordingly, in the case that an electrophotographic photosensitive member includes a photoconductive layer of the p-type, for example, when an electrostatic latent image is formed according to the above-mentioned electrophotographic process to provide a negative charge as a primary charge, to apply a sinusoidal wave a.c. corona charging as a secondary charge simultaneously with an imagewise exposure and subsequently to provide a uniform exposure, it is difficult sufficiently to neutralize and release a negative charge at a bright area of the latent image and to obtain a sufficient contrast voltage between a bright and a dark area thereof, indicating a negative value of a potential level at the bright area (-100 to -600 V). When the latent image is developed with a positive toner, it disadvantageously shows a photographic fog at the bright area.
To eliminate the above disadvantage, a process such as shown in Japanese Patent Publication No. Sho 47-26911 has already been proposed in which a secondary corona charging is provided by superposing a d.c. voltage on a sinusoidal wave a.c. voltage. With the process, a voltage-time integration value of one polarity of the a.c. voltage to be applied to a corona charger is made larger than that of the other polarity to improve the corona discharging characteristics to a desired one and a voltage level at the bright area after formation of an electrostatic latent image is controlled to a value close to zero volts. Since the voltage level at the bright area becomes substantially zero volts, the possibility of causing a photographic fog at the bright area is advantageously reduced.
However, with this process, in which a corona discharging is effected employing a sinusoidal wave a.c. voltage changing every moment, it is considered that an ion current density and a kind of ions vary as time goes, since a state not provided with a.c. discharge is caused with a certain polarity and within a certain range of voltage below a discharge initiation voltage or a corona discharge current varies every moment. Accordingly, although the presence of a corona discharge, an ion current density and a kind of ions are very important factors to the formation of an electrostatic latent image, there may be caused the trouble that these factors are not necessarily predetermined ones. This trouble may cause the stability of an image quality to be easily impaired and this is a large problem especially under an environment of high humidity. Specifically, with this process, a corona discharging becomes unstable and hence there is a great possibility that the stability and the uniformity of a potential level may not be sufficiently maintained.